Gliosis occurs in response to brain injury and is associated with many common neurological conditions such as trauma, stroke, seizure, and degenerative and demyelinative disorders. Gliosis, which is characterized by the hypertrophic and hyperplastic response of astrocytes to injury, is often regarded as an impediment to regeneration, but reactive astrocytes also possess neurotrophic properties that can support neuronal growth and axonal guidance. The molecular mechanisms that underlie the development of gliosis are not well defined, but one such mechanism may involve extracellular signal regulated protein kinase (ERK) and Raf, key members of a signal transduction cascade important in proliferation and differentiation. ATP is released upon injury and activates the ERK cascade via ATP receptors termed P2Y (G protein-coupled) and P2X (ligand-gated ion channel) receptors. ATP receptors, alone or in combination basic fibroblast growth factor (FGF2), can stimulate or inhibit astrocyte growth, depending perhaps on the type of receptor activated and the properties of the associated signaling pathway. The objective of this application is to determine how ATP receptor/ERK signaling regulates the commitment to cell cycle progression or growth arrest in astrocytes. The central hypothesis for the proposed research is that distinct types of ATP receptors regulate cell cycle progression or arrest by activating the ERK cascade for different durations and intensities and by regulating expression of different profiles of cyclins and cyclin-dependent kinase inhibitors. This hypothesis will be tested by (1) determining differences in the strength of Raf and ERK signaling regulated by P2Y and P2X receptors, (2) determining how P2Y and P2X receptors differentially regulate the temporal expression of cyclins and cyclin-dependent kinase inhibitors induced by FGF2, and (3) determining gliotic outcomes, such as proliferation, stellation, and the expression of molecules involved in axonal regeneration, that are differentially regulated by P2Y and P2X receptors on cultured astrocytes and in rat brains. This research is significant because it will provide an understanding of molecular mechanisms that can stimulate or inhibit the formation of reactive astrocytes. This may offer an opportunity to enhance the beneficial, axonal growth-promoting features of reactive astrocytes while attenuating their harmful, growth-inhibiting properties. Thus, important advances in the understanding of the mechanisms underlying gliosis may provide new approaches to restore deficits in motor skills and cognitive functions caused by brain injury. [unreadable] [unreadable]